Diorganopolysiloxanes containing triorganosiloxy groups as terminal units are usually prepared by equilibration or equilibration and condensation of cyclic and/or linear organopolysiloxanes using triorganosilanols or low-molecular-weight organosiloxanes containing triorganosiloxy groups. Such diorganopolysiloxanes containing triorganosiloxy terminal groups are used, for example, for crosslinking, i.e., curing or vulcanization in the formation of elastomers. In preparing diorganopolysiloxanes containing triorganosiloxy terminal groups by equilibration or equilibration and condensation of cyclic and/or linear organopolysiloxanes using triorganosilanols or low-molecular-weight organosiloxanes containing triorganosiloxy groups, the two terminal units of the resultant linear organopolysiloxane molecule do not always contain triorganosiloxy groups. Thus, the resultant diorganopolysiloxanes which contain Si-bonded hydroxyl groups, even in a low amount, or contain groups which are capable of condensing with such groups, when combined with reinforcing fillers form batches which may become hard or stiff on storage before final shaping and curing to form elastomers. This behavior is known, for example, as "crepe hardening". Behavior of this type can be prevented, for example, if the Si-bonded hydroxyl groups which are still present in the diorganopolysiloxanes are converted into triorganosiloxy groups.
Conversion of Si-bonded hydroxyl groups into triorganosiloxy groups by means of N-alkyl-substituted silylamines is described, for example, in U.S. Pat. No. 3,133,110 to Morehouse et al.
U.S. Pat. No. 4,725,643 to Burkhardt describes a method for preparing linear organopolysiloxanes containing triorganosiloxy groups as terminal units from linear organopolysiloxanes which contain one Si-bonded hydroxyl group in each terminal unit by condensing linear organopolysiloxanes having terminal Si-bonded hydroxyl groups in the presence of chlorophosphazene, and thereafter reacting the resultant product with hexaorganodisilazane.
It is, therefore, an object of the present invention to provide a process for preparing diorganopolysiloxanes containing terminal triorganosiloxy groups. Another object of the present invention is to provide a process for preparing diorganopolysiloxanes containing terminal triorganosiloxy groups from diorganopolysiloxanes which may already contain terminal triorganosiloxy groups after they have reached the desired mean viscosity by condensation and/or equilibration in the presence of catalysts other than chlorophosphazenes and hexaorganodisilazane and chlorophosphazenes, in which the resultant diorganopolysiloxanes are substantially free of Si-bonded hydroxyl groups or groups which are capable of condensing with the hydroxyl groups. Another object of the present invention is to provide a process for preparing diorganopolysiloxanes containing terminal triorganosiloxy units in which the hexaorganodisilazane is used in very low amounts and in the absence of another substance. Still another object of the present invention is to provide a process for preparing diorganopolysiloxanes having terminal triorganosiloxy units in which no additional agent is necessary to deactivate the chlorophosphazene and the resultant diorganopolysiloxanes do not undergo any further change in viscosity by condensation and/or equilibration as a result of the addition of the chlorophosphazene. A further object of the present invention is to provide diorganopolysiloxanes having terminal triorganosiloxy units which have a high thermal stability, are physiologically acceptable and do not discolor even at temperatures above 150.degree. C.